The present invention relates to a method for actuation and to an actuating circuit for a switch in a power factor correction circuit (power factor controller, PFC).
A power factor correction circuit usually is a step-up converter (boost converter) and comprises an inductive storage element, a rectifier arrangement, connected to the inductive storage element, for providing an output voltage, and a switch connected to the inductive storage element. The switch regulates the current drawn by the inductive storage element on the basis of the output voltage and is connected such that the storage element absorbs energy via input terminals, and is magnetized as a result, when the switch is closed, and outputs the absorbed energy to the rectifier arrangement, and is demagnetized as a result, when the switch is subsequently opened.
To control the power consumption, and hence the output voltage, such a power factor controller generates a control signal which is dependent on the output voltage and which determines particularly the lengths of the magnetization phases of the inductive storage element.
The input voltage for a power factor controller is usually a rectified mains voltage and therefore has a voltage profile in the form of the magnitude of a sine wave. In the case of a power factor controller, the current drawn will ideally be controlled such that a mean value for an input current is proportional to the applied input voltage. In an ideal power factor correction circuit, in which the energy absorbed by the inductive storage element when the switch is closed is output fully to the rectifier arrangement when the switch is opened, this can be achieved by setting the on-time to a value which is dependent on the output voltage and—when the switch has been turned-off—by turning the switch on again when the inductive storage element is free of energy or demagnetized. The power consumption is then proportional to the square of the input voltage and has a sinusoidal profile at a frequency which corresponds to twice the mains frequency.
In a real power factor correction circuit, however, losses occur which for example, are caused by a parasitic capacitance present in parallel to the switch. Such losses become even more noticeable the smaller the instantaneous value of the power consumptions, and result in distortion of the current profile of the input current over the sinusoidal profile of the mains voltage. This means that a total harmonic distortion in the input current is significantly greater than zero.
To compensate for such losses which distort the current profile, it is known to extend the on-time in comparison with the on-time which is set by the control signal.